Go back to using uintptr_t, because code that actually compiles is
infinitely less buggy than code that is theoretically correct in some
alternate universe.

The uintfptr_t type is apparently a freebsd invention, and exists only when
compiling the kernel. It's a little hard to say for sure, since it doesn't
seem to be documented anywhere except in email advice to unsuspecting and
overly-trusting souls, who then get to wear the pointy hat for blindly
following advice without investigating or testing it first.